1. Field of the Invention
The present invention is directed in general to wireless communication systems. In one aspect, the present invention relates to a method and system for directional coupling of a communications signal with minimum distortion.
2. Related Art
Communication systems are known to support wireless and wire-lined communications between wireless and/or wire-lined communication devices. Such communication systems range from national and/or international cellular telephone systems to the Internet to point-to-point in-home wireless networks. Each type of communication system is constructed, and hence operates, in accordance with one or more communication standards. For instance, wireless communication systems may operate in accordance with one or more standards including, but not limited to, IEEE 802.11, Bluetooth (BT), advanced mobile phone services (AMPS), digital AMPS, global system for mobile communications (GSM), code division multiple access (CDMA), local multi-point distribution systems (LMDS), multi-channel-multi-point distribution systems (MMDS) and/or variations thereof.
Depending on the type of wireless communication system, a wireless communication device (such as a cellular telephone, two-way radio, personal digital assistant (PDA), personal computer (PC), laptop computer, home entertainment equipment, etc.) communicates directly or indirectly with other wireless communication devices. For direct communications (also known as point-to-point communications), the participating wireless communication devices tune their receivers and transmitters to the same channel or channels (e.g., one of the plurality of radio frequency (RF) carriers of the wireless communication system) and communicate over the tuned channel(s). For indirect wireless communications, each wireless communication device communicates directly with an associated base station (e.g., for cellular services) and/or an associated access point (e.g., for an in-home or in-building wireless network) via an assigned channel. To complete a communication connection between the wireless communication devices, the associated base stations and/or associated access points communicate with each other directly, via a system controller, via the public switched telephone network, via the Internet, and/or via some other wide area network.
Wireless communication devices typically communicate with one another using a radio transceiver (i.e., receiver and transmitter) that may be incorporated in, or coupled to, the wireless communication device. The transmitter typically includes a data modulation stage, one or more intermediate frequency stages and a power amplifier. The data modulation stage converts raw data into baseband signals in accordance with a particular wireless communication standard. The intermediate frequency stages mix the baseband signals with one or more local oscillations to produce RF signals. The power amplifier amplifies the RF signals prior to transmission via an antenna. In direct conversion transmitters/receivers, conversion directly between baseband signals and RF signals is performed. The receiver is typically coupled to an antenna and includes a low noise amplifier, one or more intermediate frequency stages, a filtering stage and a data recovery stage. The low noise amplifier receives inbound RF signals via the antenna and amplifies them. The intermediate frequency stages mix the amplified RF signals with one or more local oscillations to convert the amplified RF signal into baseband signals or intermediate frequency (IF) signals. The filtering stage filters the baseband signals or the IF signals to attenuate unwanted out of band signals to produce filtered signals. The data recovery stage recovers raw data from the filtered signals in accordance with the particular wireless communication standard.
As the use of wireless communication devices increases, many wireless communication devices will include two or more radio transceivers with two or more antennas, where each radio transceiver is compliant with any of a variety of wireless communication standards and may be used with the exemplary communication systems described herein, including Bluetooth, IEEE 802.11 (a), (b), (g) and others. For instance, a computer may include two radio transceivers, one for interfacing with an 802.11a wireless local area network (WLAN) device and another for interfacing with an 802.11g WLAN device. In this example, the 802.11g transceiver operates in the 2.4 GHz frequency range and the 802.11a transceiver operates in the 5 GHz frequency range.
Many of the current transceivers comprise directional couplers to provide a measurement of transmitted power for quality and regulatory purposes. For example, current regulations promulgated by the U.S. Federal Communications Commission (FCC) require precise measurement of RF emissions from wireless communications devices.
A directional coupler is a linear, passive, multi-port network, consisting of a pair of electromagnetically coupled signal conducting “lines” or structures such as strip lines or transmission lines. One of the pair of lines is a “main signal line” that connects an input port of the coupler to an output port. The other of the pair of lines is an “auxiliary signal line” that is connected to at least one measurement or utilization port. The auxiliary line is coupled to the main line through a “coupling region” where the lines are in close proximity to each other. A radio frequency (RF) signal applied to the main line induces a signal in the auxiliary line. This signal can be measured to provide an indication of the forward transmitted power or the reflected transmitted power. An example of a directional coupler is disclosed in U.S. Pat. No. 6,686,812,entitled “Miniature Directional Coupler,” issued to Gilbert et al. on Feb. 3, 2004, which is incorporated by reference herein for all purposes.
Directional couplers are generally either 3-port unidirectional couplers and 4-port bi-directional couplers. A unidirectional coupler consists of a main line and an auxiliary line, which can be internally terminated in the coupler at one end with the other end providing the coupled output. A bidirectional coupler is similar to the unidirectional coupler with the exception that both ends of the auxiliary line provide coupled outputs. Thus the bi-directional coupler can be used for simultaneously monitoring both the forward and the reflected power.
Forward transmitter power may be monitored to determine transmitter output power and efficiency. A directional coupler can perform as a measurement tool that samples a small portion of the radio frequency energy traveling through the main line between a signal source and a load, for instance. This energy can travel “forward” from a signal source such as a transmitter to a load such as an antenna and/or the energy can be reflected in “reverse” from the antenna to the transmitter.
One of the problems associated with directional couplers is signal degradation associated with distortion created by the circuit components used to measure the transmitted power. For example, many directional couplers use a diode detector and a smoothing capacitor to measure forward transmitted power. The diode is a non-linear device that can create distortions at frequencies corresponding to harmonics of the fundamental carrier frequency of the transmitted signal. As will be understood by those of skill in the art, combinations of the harmonic frequencies can cause intermodulation interference that can degrade or destroy the transmitted signal. In view of the foregoing it is apparent that there is a need for an improved directional coupler that significantly reduces the signal degradation problems associated with distortion caused by circuit elements used to measure the transmitted power.